1. Field of the Invention
The present invention relates to a FLOTOX type EEPROM, particularly, to an improvement in a method of manufacturing a thin oxide film, hereinafter referred to as "tunnel oxide film", through which flows a Fowler-Nordheim current in a programming stage of information.
2. Description of the Related Art
In accordance with a marked progress achieved in recent years in the miniaturization of the memory cell of FLOTOX type EEPROM, a tunnel region in which a tunnel oxide film is formed tends to be formed such that the edge portion of the tunnel region overlaps with the adjacent field oxide film, as shown in FIG. 1. Incidentally, FIG. 1 is a plan view showing a single memory cell comprising a tunnel region 11, a field oxide film 12, a floating gate 13, a source region 14 and a drain region 15.
FIGS. 2A to 2D are cross sectional views along the line A--A' shown in FIG. 1, showing a conventional method of forming a tunnel oxide film of a memory cell in the case where the tunnel region overlaps with the edge portion of the field oxide film as shown in FIG. 1.
In the first step, a channel stop 22 is formed in a p-type silicon substrate 21 by selectively implanting, for example, boron ions, as shown in FIG. 2A. A field oxide film 23 serving to isolate element regions is formed on the channel stop 22. Then, impurity ions, e.g., As ions, are implanted into the silicon substrate 21 within a tunnel region so as to form an n-type impurity region 24 within the silicon substrate 21 in the tunnel region. After formation of the n-type impurity region 24, a relatively thick oxide film 25 having a thickness of about 400 .ANG. is formed over the entire element region, followed by coating the entire silicon substrate 21 with a resist film 26, as shown in FIG. 2B. In the next step, the resist film 26 is exposed to light and, then, patterned such that the resist film 26 is selectively removed from a tunnel region 27. It should be noted that the tunnel region 27 is positioned to overlap with the edge portion of the field oxide film 23. The particular construction is advantageous in terms of miniaturization of the element.
Then, the oxide film 23 positioned in the tunnel region 27 is etched with NH.sub.4 F using the resist film 26 as a mask, as shown in FIG. 2C. Further, the resist film 26 is removed, followed by forming a relatively thin tunnel oxide film 28 having a thickness of about 100 .ANG. by, for example, thermal oxidation in the tunnel region 27, as shown in FIG. 2D. Still further, a polysilicon film about 4000 .ANG. thick is formed on the tunnel oxide film 28, followed by patterning the polysilicon film so as to form a floating gate 29.
In the conventional method described above, the tunnel region 27 is positioned to overlap with the field oxide film 23, giving rise to a difficulty. Specifically, in the step of etching the oxide film 25 in the tunnel region 27 in preparation for the formation of the tunnel oxide film 28, the edge portion of the field oxide film 23 is etched together, leading to recession in the edge of the field oxide film 23. It follows that the channel stop 22 is partially exposed to the substrate surface, as shown in FIG. 2C. This brings about a serious problem. Specifically, even if a high voltage is applied to the n-type impurity region 24 for withdrawing electrons from the floating gate 29 in the programming step of information, the holes formed by the band-to-band tunneling within the n-type impurity region 24 leak into the silicon substrate 21 through the channel stop 22. In other words, it is difficult to apply an electric field of high intensity to the tunnel oxide film 28, making it difficult to permit flow of Fowler-Nordheim current.